The .348 case is a little on the fat side. My guess is what happened here is a 'smith used a .348 reamer in the .43 Spanish chamber so the winchester case would go in, then it was up to the owner to fire-form the case. Like with a primed case, 15 gr of Unique and a load of cornmeal over it. The resulting case would work, although it would be shorter than the old .43. My brother has some .43 cases that were turned down on a lathe from .348 brass. It is close but no cigar.
I tried to run a .348 Win case through my .43 sizing die when I was a teenager. Took me a week to get it out.
The rifle sure looks like Modelo Argentino 18xx. A lot of those were broached out at the arsenal when Peron was running things. Those guns take a larger bullet.
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